Isolated bone Cells and bone organ cultures are being used to determine the relationship between calcium fluxes and later events in the bone-resorbing actions of parathyroid hormone and vitamin D metabolites. Models of the calcium mediated effects are provided by the divalent cation ionophores and their bone-resorbing effects and interactions with PTH and vitamin D metabolites are being characterized. A bioassay for 1,25-(OH)2 vitamin D based on bone resorption in vitro is being applied to clinical conditions with abnormal calcium metabolism, primarily sarcoidosis in order to define the underlying defects in vitamin D metabolism.